High Energy Lasers

As I understand it, at a certain field strength you damage your optical media, but I know high power cw lasers do exist, so I wonder:

1) How do you shutter a high power laser?
2) Is it possible to join two laser beams. The only way I see would be with a polarizing beam splitter in reverse, but you can only do that once and the medium hast to survive.
3) Is the key to success to tune all your mirrors exactly to the laser frequency?